My invention is adapted especially for use in a rack-and-pinion power steering gear, although it is capable of being used also with other kinds of fluid pressure assisted steering gear mechanisms.
It is conventional practice in prior art steering gear mechanisms to use a rotary valve in a fluid circuit between a power steering pump and a fluid pressure operated motor that is adapted to deliver steering power assist to the steering linkage of an automotive vehicle. The pump in such arrangements usually is a positive displacement pump. A pressure valve control and a flow control valve cooperate with the pump to form a pump assembly.
The flow control valve is adapted to bypass fluid from the discharge side of the pump to the pump reservoir on the inlet side of the pumping chambers, the amount of bypass flow directed from the flow delivery passage to the inlet side of the pump increasing as the driven speed of the pump increases. The flow control valve responds to changes in pump speed either to increase the bypass flow or to decrease it depending upon whether the pump speed increases or decreases. Thus, the effective output flow of the pump is constant throughout a large range of pumping speeds.
The constant flow that thus is delivered to the steering valve results in a steering pressure in the fluid motor for the steering gear mechanism that is proportional to the reciprocal of the square of the effective flow area established by the steering valve. This relationship between steering pressure and valve flow area is expressed by the following equation: ##EQU1## where Q is the flow per unit of time and A is the effective flow area. If it is assumed that Q is a constant throughout the operating range of speeds of the pump, the equation can be expressed in terms of area and a new constant K.sub.1 as follows: ##EQU2##
In rotary valves for power steering gear mechanisms of this kind, an inner valve member is surrounded by a valve sleeve, the sleeve being disposed in a steering gear valve housing. A driver-operated steering shaft functions as a torque input shaft for the steering gear mechanism and is connected to the inner rotary valve element. The sleeve is connected mechanically to a drive pinion for a rack-and-pinion steering gear mechanism. The pinion is connected directly to the steering torque input shaft through a torsion bar which flexes through an angular displacement that is directly related to the magnitude of the steering torque. Thus, when a given steering torque is applied to the input shaft, the torsion bar will be deflected, which results in a relative rotary displacement of the inner valve member with respect to the valve sleeve.
This valve action changes the flow area for the fluid supplied by the pump. The flow rate of the fluid, as mentioned previously, is constant throughout the pump speed range. As a result of the constant flow and the variable area, the steering pressure changes in accordance with the formula expressed above.
Provision is made for distributing the variable steering pressure to opposite sides of a fluid motor in order to obtain a power assist.
In steering gear mechanisms of this kind, the pump is continuously subjected to a pressure differential because of the requirement that the discharge flow be maintained at a useful value that is high enough to provide the required steering assist under a variety of steering conditions. The pump thus creates a relatively large parasitic power loss for the engine because it is driven by the engine crankshaft.
A belt and pulley usually are used in a torque transfer drive between the pump and the crankshaft; but in some instances, the pump is driven directly by the engine crankshaft. The relatively large parasitic losses involved in driving the power steering pump is one of the disadvantages that I have overcome with my present invention. I also have simplified the overall power steering system and have adapted it for packaging in the powertrain compartment of a vehicle with an economy of space that would not be possible with a conventional power steering system.